I found this an interesting site:
LBL is the old Lawrence Berkeley National Lab (that used to do nuke work, IIRC). They have a section devoted to Heat Islands.
So looks like someone is very well aware that heat islands exist, are a large impact on thermometers, and don’t have much or anything to do with CO2… Golly.
I’ve linked into their publications section where folks can go browsing for interesting reads…